Doc: Spelling mistake and grammatical correction in CONTRIBUTING.md file

[krysclarke]

Motivation / Problem

There was a typo and a grammatical error in the "Project Goals" section, as raised in the OpenTTD forums at https://www.tt-forums.net/viewtopic.php?t=90344

Description

As mentioned in the forum thread, in the "Project Goals" section of CONTRIBUTING.md, I noticed two errors - a spelling mistake ('extent' used, 'extend' meant) and a grammatical error ('that is what everyone brought here', used, 'that is what brought everyone here'

Limitations

No limitations known, as this affects a documentation file, nothing in-game.

Checklist for review

Some things are not automated, and forgotten often. This list is a reminder for the reviewers.

• The bug fix is important enough to be backported? (label: 'backport requested')
• This PR touches english.txt or translations? Check the guidelines
• This PR affects the save game format? (label 'savegame upgrade')
• This PR affects the GS/AI API? (label 'needs review: Script API')
  • ai_changelog.hpp, gs_changelog.hpp need updating.
  • The compatibility wrappers (compat_*.nut) need updating.
• This PR affects the NewGRF API? (label 'needs review: NewGRF')
  • newgrf_debug_data.h may need updating.
  • PR must be added to API tracker